Dielectric material



r`rune 2.2, 1943. H, FRUTH 2,322,353

DIELECTRIC MATERIAL Filed NOV. 4, 1939 DWS/TY Patented June 2 2, 1943'DIELECTRIC MATERIAL Hal F. Fruth, Riverside, Ill., assignor. to WesternElectric Company, Incorporated, New York, N. Y., a corporation of NewYori:

'Application November 4, 1939, Serial No. 302,835

5 Claims.

This invention relates to dielectric material for use in electriccondensers of the type having plates of conducting material Aseparated.by a solid dielectric medium, and to methods for making such condensers.

An object of this invention is the production of an electric condenserof economical construction having high capacity values in proportion toits volume and improved operating characteristics.

Condensers ofthe type comprising plates or electrodes of metal and aninert intermediate dielectric, such as -sheets of impregnated paper ormica, have been used extensively in electrical apparatus. In thesecondensers, `the effective capacity is inversely proportional to thedistance between the electrodes and directly proportional to thedielectric constant 'of the insulating medium between the electrodes,other factors being equal. It is, therefore, desirable to use aninsulating material with a high dielectric constant and to apply thematerial in a very thin layer in order to .provide high unit capacityand thus minimize the required electrode area and cost of the condenser.

Certain crystallinel materials exhibit very high dielectric constants incomparison with paper and mica. For example, titanium dioxide, or

foil coating, winding the coated foil into a condenser, and impregnatingthe condenser vwith an agent such as a chlorinated naphthalene, toregulate and improve its temperature-capacity coefcient.

Following is a more detailed description of the invention, taken inconjunction with the appended drawing, in which Fig. 1 is a perspectivedrawing of a condenser embodying certain features of the invention;

Fig. 2 is an enlarged fragmentary view of the condenser shown in Fig. 1;and l Fig. 3 is a chart showing certain properties of condenserinsulation made in accordance Jwith the invention.

The type o f condenser to which the invention is primarily adaptedcomprises conducting plates or electrodes with suitable terminalsattached thereto and insulation between the plates. The electrodes maybe flat or they may be formed of flexible metal foil rolled or woundinto a compact body. A wound o1' vrolled type condenser IIJ is shown inFig. l. This structure comprises two sheets of metal foil separt'd byinsulation and wound irto an oval body. Each sheet of foil has Rutile,crystals have a specific inductive capacity as high as 170 along theirprincipal or major axis and around 90 along their transverse axis. Arandom mixture of titanium dioxide particles has a dielectric constantaround 110. Howeventhe utilization of -these inherently advantageousproperties in electric condensers has been subject to certainlimitations. One primary objection has been the difiiculty in applyingthe titanium dioxide to the condenser electrodes in a sufilciently thinlayer. Ceramic or fused bodies of titanium dioxide have been preparedfor this purpose, but

their use is limited to constructions wherein the insulation can beapplied in its ultimate shape and form. Also, the ceramic bodies arenecessarily made relatively thick, in vorder to provide sufllcientstrength for handling in their unred form, and the low area-volume ratioof these bodies reduces the condenser capacity proportionately.

In accordance with one embodiment of this invention, an eillcientlelectrostatic vcondenser having a high capacityl relative to its volumeis4 provided by coating flnelywdivided particles of titanium dioxidewith an organic binder such as cellulose acetate, applying the. coatedparticles in a thin layer on metal foil, hot calendering the coated foilto coalesce the binder and density the a terminal Il and I2 electricallyconnected thereto. These general condenser constructions are well knownand this invention relates to the com; y position, preparation,application and treatment of the insulation used between theelectrode..Y layers.

To prepare the insulation, titanium dioxide is rst ground or pulverizedinto ne particles. The. optimum particle size depends somewhat on theservice requirements of the condenser, but, in general, particles thatwill pass through a 320 mesh screen are satisfactory.

The une particles of titanium dioxide are next coated with an organicmaterial that serves as a binder. Various organic materials aresatisfactory for this purpose including certain cellulose derivatives,some types of shellac, tung oil and phenol resin. In general, the bindershould have good dielectric properties, such as a high dielectricconstant and aA low power factor, be

'chemically stable, 4and be available in a form tween the electrodes.

to the size and shape oi' the electrodes.

a suitable apparatus, such as a ball mill. Agitation of the mixture inthe mill is continued until the particles are completely coated with thecellulose acetate.

. The proportion of binder to be used depends upon the size of theparticles, the construction and service requirements of the condenserand the type e binder employed. It is desirable to completely cover eachparticle without providing a great excess of binder. The effect ofvariations in binder content is illustrated on the chart shown in Fig.3. The two curves A and B on this chart show the dielectric constant 'ofmixtures prepared with various proportions of binder. As demonstrated bythe curves, the dielectric constant increases rapidly with reduction ofbinder content and for that reason it is desirable to use only enoughbinder to adequately coat the particles. A mixture containing aboutcellulose acetate by volume and the remainder titanium dioxide particlessuillciently small to pass ,through a 320 kmesh screen has been usedsuccessfully and, in general, a mixture containing from 10% to 50% ofbinder by volume and the lremainder titanium dioxide particles of propermethods that are adapted primarily to the structure of the electrodes.For use with tiatplates or electrodes, the mixture can be molded intoflat bodies under heat and pressure, the thermoplastic coating on theparticles serving tc bind them in a unitary body. For use with eitherhat or rolled type electrodes, the mixture can be applied on the surfaceof a paper or other sheet material and the coated sheet then insertedbe- However, it is diillcult to produce the insulation in sullicientlythin bodies in either of these forms and the best results are obtainedby applying the mixture directly on the electrodes. y

The exact electrode coating method is adapted For insulating fiatelectrodes, the mixture, in a pastelike consistency, can be painted orspread evenly on the electrodes with a suitable tool. Metal foil usedfor coiled or rolled condensers can be coated conveniently by conductingthe foil through a bath of the mixture, thinned with solvents or heat. Asuillcient quantity of cellulose acetate solvent, such as acetone, isadded or sumeient heat is applied to make the cellulose acetate coatingon the particles plastic and cause them to adhere to the Ioil. Thespeedof the foil through the mixture is regulated to insure formation ofa continuous coating of the particles on both faces of the foil.

After the` electrodes arey coated, the mixture is solidiiled, byevaporation of the solvent or by cooling, and the coating is thentreated to increase its density and improve its uniformity. This isaccomplished by calendering the coated electrodes between heatedrotating rolls positioned a iixed distance apart or by pressing thecoated electrodes between the heated platens of a press. The combinationof heat and pressure iully coalesces the particle coatings, compactsthetoil coating to reduce the presence ci' voids therein, and alsoimproves the uniformity of the coating thickness. v

TheI coated electrodesare then assembled or wound into a condenser. I 'oform the rolled con` denser shown in Fig. 1, a strip of the coated metalfoil is placed upon a strip oi' uncoated metal` i'oil ,of the samethickness. The coated and uncoated foil strips thus assembled are woundinto a body on a conventional condenser Winding machine. These machinesembody a rotating mandrel, which engages the end portions of the twofoils and then winds them into the condenser structure.. During thewinding operation, a terminal Il and I2 (Fig. l) is connectedelectrically to each oi' the foil strips in any suitable manner.

In the wound structure, the coated and the uncoated foils are spacedapart and insulated throughout the structure by the coatings on thecoated foil. As shown in Fig. 2, the alternate layers formed by theuncoated foil l5 engage the insulating coatings I6 on the two surfacesoi.

the coated foil I1. By using a single, compact and thin insulating lm ofhigh specific dielectric value between the turns or layers of foil inthis manner, a structure having a high capacity in proportion to theelectrode area is produced. Aluminum foil is used generally forcondensers of this type. For some applications, it is tiene1 iicial toprovide a coating of aluminum oxide on the bare or uncoated foil l5.This oxide coating is preferably applied by the anodic oxidationprocess, which is well known. When anodically oxidized aluminum foil isused, the oxide layer engages the titanium dioxide coating on theadjacent foil to provide a particularly eiective in sulation for highpotential condenser service.

.The condenser in this form; with or Without the oxide coating on onefoil, is satisfactory for certain fields of service. It has goodproperties at low operating temperatures, but its capacity rating isreduced as the temperature of the condenser is increased. In some cases,the capacity value was reduced as much as 20% when the temperature ofthe condenser was raised from F. to 110 F.

To improve this characteristic, the wound structure is impregnated withan agent toadjust the temperature-capacity coefficient. Chlorinatednaphthalene is the preferred agent and hydrogenated castor oil is alsosuitable for this purpose. Both of these materials are solids at roomtemperatures and they are applied to the condenser in hot or liquidform. ln the impregnating process, it is preferable to iirst evacuateall air from the condenser and then completely immerse the condenser inthe agent. This causes the agent to Illl any voids in the ioilj coating,as well as any minute spaces between the coating and the foil. 'I'heimpregnating agent and the titanium dioxide-binder coating cooperativelyprovidea temperature-capacity coefiicient that is slightly positive withthe result that the capacity of thecondenser is actually increased atelevated temperatures. The eil'ect of the impregnani; on the dielectricconstant of the titanium dioxide-binder mixture at a temperature of 70F. is shown in Fig. 3, where line A represents values for a condenserimpregnated with hydrogenated castor oil and the line B shows the valuesfor al condenser impregnated with chlorinated naphthalene. v

` Flat type condensers can also be made by this general procedure. Theilat electrodes can be coated by immersion in a fluid mass of the coatedtitanium dioxide particles and cellulose acetate, followed by drying andhot pressing to coalesce and density the coating. The iiat coatedelectrodes are then assembled with bar or oxide coated electrodes andthe assembly is then impregnated with the temperature-capacitycoetlicient adjusting agent. l

The above described'procedures are satisfactory 4when a thermoplasticbinder is used. If a thermosetting binder material, such as a phenolicbase resin is employed, a nal and more prolonged heating oi' the coatedelectrode is necesing layer between electrodes, as above described,

fil

,positive temperature coeillcient of dielectric d is preferred becausethis construction provides s the closest electrode spacing. Thispreferred lulose acetate, and a wax-like material selected from thegroup consisting of hydrogenated castor oil and chlorinated naphthalene,said mixture having a dielectric constant greater than ilfty in thesolid state and a positive temperature coetllcient of dielectricconstant.

2. A dielectric material for use in electrical apparatus such aselectric condensers comprising about cellulose acetate by volume, iinelydivided particles of titanium dioxide dispersed in the celluloseacetate, and a wax-like material selected from the group consisting ofhydrogenated castor oil and chlorinated naphthalene in the voids in thecellulose acetate and the voids between the cellulose acetate and theparticles,

said dielectric material having a dielectric con- A stant in the solidstate greater than fifty and a constant.

3. A dielectric' material for use in electrical vapparatus such aselectric condensers comprising from to 90% of finely divided titaniumdioxide particles suillciently small to pass through a screen on theorder of 300 mesh, a`

cellulose acetate binder for said particles, and a sufilcient quantityof wax-like material selected from the group consisting of hydrogenatedcastor oil and chlorinated naphthalene to iill the voids in said binderand the voids between the binder and particles, said dielectricmaterialhaving a dielectric constant greater than fifty in the solidstate and being capable of providing a positive temperature-capacitycoeilieient when incorporated in an electric condenser.

4. A dielectric material for use in electrical apparatus such aselectric condensers comprising a mixture of titanium dioxide particles,cellulose acetate and hydrogenatd castor oil, said mixture having adielectric constant greater than fty in the solid state and a positivetemperature coefficient of dielectric constant.

5. A dielectric material for use in electrical apparatus such aselectric condensers comprising a mixture of titanium dioxide particles,cellulose acetate and chlorinated naphthalene, said mixture lhaving adielectric constant greater than fty in the solid state and a positivetemperature coeillc'ient oi' dielectric constant.

HAL F. FRU'I'H.

